The present invention relates to the use of N,N'-bis-salicyloyl-hydrazine as a metal deactivator for organic materials which are in contact with copper or contain copper or copper ions.
Many organic materials which currently are used in electrical technology for insulation purposes, for example, polymers such as polyoxymethylene, polyamide and unsaturated polyester resins, and particularly polymers such as polyolefins, are subject to accelerated thermo-oxidative aging in the presence of copper. This disadvantageous property considerably impairs the electrical and mechanical properties of these materials in long-term use. The detrimental effect of copper on these materials is particularly aggravated at elevated temperatures since the aging rate of the polymers increases steeply with increasing temperatures.
Cross-linked polyolefins, which are used increasingly as insulating material for cables and wires, are subject to greatly accelerated aging in the presence of copper and must, therefore, be protected effectively against the oxidation-accelerating influence of copper. According to experience, this can be done by placing a foil as a separating layer between the copper conductor and the insulation or by using tinned conductors, thereby avoiding direct contact of the copper with the insulation. Such measures, however, are expensive and very demanding from a manufacturing point of view. Thus, if a foil is used as a separating layer, only low production rates are possible, particularly with small conductor cross sections.
Another approach for meeting the desired requirements as to quality and allowable temperature stress of polymer materials in contact with copper is the use of so-called "copper deactivators" which inhibit the oxidation-accelerating effect of copper at elevated temperatures. Stabilizing polymer materials through the use of copper deactivators is therefore a cost-effective measure.
It is known from U.S. Pat. No. 3,849,492 to use metal deactivators of the N,N'-bis-salicyloyl hydrazine type for this purpose. In this connection, multiple alkyl-or alkoxy-substituted derivatives of the base compound are said, in particular, to be effective for stabilizing polyolefins against the deleterious effect of copper and other transition metals.
From German Pat. No. 27 03 558, it is known (see column 2, lines 49 to 65) that for durable stabilization of polymers in contact with copper, a combination of N,N'-bis-salicyloyl hydrazine as a metal deactivator and oligomeric 2,2,4-trimethyl-1,2-dihydroquinoline as an oxidation inhibitor is commercially advantageous. This stabilizer combination also has been found to be particularly effective for stabilizing cross-linked polyolefins. The effectiveness of the unsubstituted N,N'-bis-salicyloyl hydrazine as a metal deactivator was determined on the basis of extensive investigations on model conductors and from aging tests on commercial products.
In the mentioned stabilizer combination, the product serving as the metal deactivator was obtained directly through reaction of commercial-grade salicylic acid hydrazide with commercial-grade salicylic acid with the addition of commercial-grade thionyl chloride and pyridine in commercial-grade chlorobenzene, and was present, after purification by washing with alcohol, with a purity of 99% (see also British Pat. No. 1 398 360).
A metal deactivator of the type mentioned and other additives in a polymer material such as a polyolefin for wire and cable insulation are worked-in on a commercial scale by conventional mixing methods. In cable and wire technology, it is preferred that so-called "concentrates", consisting of a metal deactivator or other additives and polymer material, are prepared first. The concentrates are then processed into insulating mixtures with the desired concentrations of metal deactivator and the further additives by mixing them into further polymer material.
In the manufacture of the concentrates with the metal deactivator prepared in this manner on a commercial scale it has been found that eye irritation or damage can occur in persons concerned therewith. These eye irritations or damage could be traced to the metal deactivator used.
To avoid eye irritation or damage, it has, therefore, been attempted to use as a metal deactivator an N,N'-bis-salicyloyl hydrazine which is prepared by known methods from salicylic-acid chloride and hydrazine or from salicylic-acid chloride and salicylic-acid hydrazide, which is purified by recrystallization, and in which the mass content of chlorine is .ltoreq.0.002%, the mass content of sulfur is .ltoreq.0.07% and the overall acidity (monobasic acid) is .ltoreq.0.4 mmol/kg. These maximally permissible impurity contents were determined by animal tests in which extracts of N,N'-bis-salicyloyl hydrazine products of different purities in isotonic common-salt solution were applied to rabbits over a period of 3 weeks (test method 1), on the assumption that only the components soluble in the body fluid have a harmful effect.
With a modified, drastically intensified test method (test method 2), in which the solid substance in question repeatedly was placed directly in the eye of test animals (rabbits) at relatively short time intervals, i.e., days, heavy irritation was found in the case of such a product. This test method, which also was carried out in the following context under intensified conditions, assumes that the metal deactivator in solid form, for example, as dust, can get into the eye.